Carbon dioxide (hereinafter “CO2”) capture and sequestration from coal fed power plants presents a large challenge. While there are existing technologies, they are capital and energy intensive. In an integrated Gasification Combined Cycle (hereinafter “IGCC”) plant, coal is gasified and a syngas which contains CO2, as well as a variety of other components, is produced. Typically, the CO2 from the syngas is removed by solvents before being sent to the gas turbine where it is burned to produce electricity. The cost of CO2 removal increases the cost of the electricity produced. An industry wide goal has been set that the cost increase should not exceed 30%. To meet this goal new innovations for CO2 capture are required.
Today coal or solid bio-mass is gasified in a variety of gasifiers. For pre-combustion CO2 removal, CO2 is removed from the CO2/hydrogen mixture obtained after shifting carbon monoxide (hereinafter “CO”) rich syngas. CO2 typically forms from about 40% to about 60% of the syngas after CO is shifted to CO2. The CO2 separation is usually performed by utilizing solvents such as RECTISOL®, SELEXOL® or amines. These processes produce CO2 at a low pressure, close to atmospheric pressure. A large amount of power is required to compress CO2 to 150 bar, the pressure required for sequestration. Accordingly, these processes are capital as well as energy intensive.
A variety of hydrogen selective membranes such as palladium membranes to concentrate the CO2 from the syngas stream have been developed. For example, U.S. Pat. No. 7,175,694 describes such a membrane. These membranes intensify the CO shift and produce a CO2 rich stream at high pressure. The CO2 free hydrogen produced by membranes is at low pressure and is then compressed for use in the gas turbine. There is a significant loss of hydrogen (hereinafter “H2”) and CO products left in the CO2 stream, thereby affecting the efficiency of the process. Accordingly, there is a need to overcome this efficiency issue.
In a 1997 Elsevier article (“Water Gas Shift Reactor For CO2 Control In IGCC Systems: Techno-Economic Feasibility Study”), Bracht et al. describe a system with a membrane shift reactor on a gasifier syngas. The solution proposed for residual gas treatment was a catalytic burner which requires the use of oxygen (hereinafter “O2”) and the residual CO and H2 are valued at best as fuel.
As noted above, while there currently exists a number of processes to capture and sequester CO2 from coal fed power plants, these processes are not without limitations and/or problems. The present invention provides an alternative way to treat this residual gas which overcomes many of the limitations/issues associated with these prior art processes.